A flyback switch power supply is a kind of an isolated switch power supply, which is widely used in AC/DC and DC/DC conversion and provides insulation and isolation between an input stage and an output stage. The flyback switch power supply includes a main power switch tube, a transformer, and a secondary-side rectification tube, and the transformer includes a primary-side winding and a secondary-side winding. The main power switch tube is connected with the primary-side winding, and the secondary-side rectification tube is connected with the secondary-side winding. A primary-side control circuit controls a switching state of the main power switch tube, and the secondary-side rectification tube located at the secondary-side is turned on to freewheeling after the main power switch tube is turned off.
An active clamp topological structure is to connect a capacitor with a drain of the main power switch tube in the flyback switch power supply, a switch tube is connected between the drain of the main power switch tube and the capacitor, and the capacitor is connected with an input power supply. The main power switch tube is turned off after being turned on, at the moment when the drain is turned off, the instantaneous spike and high-order harmonic of a turn-off waveform are coupled with the power supply through the capacitor to achieve an objective of clamping a voltage across the drain of the main power switch tube, which reduces a turn-off loss of the main power switch tube, thereby reducing a power loss of the switch power supply.
FIG. 1 shows an active-clamp flyback circuit. When a system is working in a boundary conduction mode (BCM) or a discontinuous conduction mode (DCM), at the beginning of each switching period, a magnetizing current is close to zero. In the prior art, turn-on and turn-off of the switch tube MA are generally controlled by a manner as shown in FIG. 2. The switch tube MA is turned on at any time during the main power switch tube M0 is turned off, and the switch tube MA is not turned on during the main power switch tube M0 is turned on.
LM is a magnetizing inductor in the primary-side winding. In the prior art, a control way as shown in FIG. 2 is used to control the turn-off of the switch tube MA. As there is no precise control for the turn-off moment of the switch tube MA before the main power switch tube M0 is turned on, a voltage across a common end of the main power switch tube and the primary-side winding may be relatively high when M0 is turned on, resulting in a relatively high energy loss.